1. Field of the Invention
The present invention relates to packing bags particularly suitable for use for light-sensitive materials and particularly to packing bags having improved physical strength, light-intercepting properties and antistatic properties.
2. Description of the Prior Art
Generally, packing bags which completely intercept light are used for packing goods whose value is destroyed or damaged upon exposure to light, such as light-sensitive materials. Such packing bags must have sufficient physical strength, that is, tensile strength, tearing strength and bursting strength, depending on the size or weight of the goods to be packed. Further, in packing light-sensitive materials in the packing bags, the bags must have a heat-sealing property, because it is necessary to completely seal the opening by heat-sealing to ensure light-interception and moisture proofing properties. In addition, where the packing bags are to be used for photographic light-sensitive materials, the packing bags must have antistatic properties in order to prevent generation of static charges due to the friction of the bag with the photographic light-sensitive material, since discharge of the static charges deleteriously affects photographic light-sensitive materials.
Hitherto, as packing bags whose properties as described above, such as light-intercepting property, physical strength, heat-sealing property or antistatic properties, etc., are statisfactory, laminates composed of a low density polyethylene film containing carbon black or pigments and paper, aluminum foil or cellophane have been widely used.
Such prior packing bags, however, have the disadvantages that they have poor physical properties such that tearing can occur or openings form during the packing operation or their size in easily changed due to stretching.
In order to eliminate such disadvantages, it is necessary to increase the thickness of the material from which the packing bags are made and, consequently, the resulting packs become bulky, heavy and expensive.